Fiber optic gyroscopes (“FOGs”) utilize a coiled length of fiber optic to detect minute amounts of rotation. The output of a light source is split whereby two light beams are created and directed into the opposite ends of a coiled length of fiber optic cable. When the FOG is rotated, the light beam traveling against the direction of rotation will experience a shorter optical path than the light beam traveling in the direction of rotation, resulting in a phase shift between the two beams. This phase shift can be measured through various means, including interferometry, to determine the magnitude of the rotation experienced by the FOG.
Recent FOG developments have focused on system miniaturization and cost reduction for use in many applications, including tactical weapon systems and inertial guidance systems. Reducing the size of the fiber optic gyroscope, without sacrificing the performance, requires that considerable attention be given to the coil of wound optical fiber that comprises the sensor. This is because reducing the size of a fiber optic coil made using conventional winding configurations would greatly increase the number of “crossover” sites.
Crossover sites are the places in a conventional winding scheme (i.e., one in which fibers are wound on a spool axially in a layer, and then subsequent layers build up on the first layer) at which a fiber in one layer crosses over a fiber of a lower layer at an angle that puts stress on the fiber. These crossover sites have been found to cause random polarization cross-coupling in the single mode fiber coil that substantially degrades the fiber optic gyroscope technology performance, causing polarization non-reciprocal (“PNR”) bias errors in the depolarized FOGs.
In the past, navigation-grade performance FOG designs have required the use of relatively expensive polarization-maintaining (“PM”) fibers. Eliminating crossovers allows for an inexpensive single mode (“SM”) fiber solution, which reduces the overall cost of the FOG. Due to the significant cost advantage of SM fibers, depolarized interferometric FOGs have great cost-lowering potential.
Another problem with conventional FOGs is the presence of time-varying thermal gradients, which are a large source of FOG error. Time-varying thermal gradients result in a phenomenon known as the Shupe effect, a phase shift due to time varying temperatures experienced by different segments of the coil, and resulting in increased bias. Reducing varying thermal gradients requires complex, costly and time-consuming winding patterns that ensure that the phase shifts are symmetrical and do not result in bias. These types of winding patterns have not been compatible with automatic, high-speed winding systems.
A crossover-free and thermally-symmetric method of winding fiber optic sensor coils was disclosed in Ruffin, U.S. Pat. No. 5,781,301 (“the '301 patent”). An explanatory illustration of the method disclosed in the '301 patent is provided at FIG. 1. This method eliminated traditional crossovers by winding the fiber in radial outside-in and inside-out layers that are stacked, instead of winding axially in overlapping layers as has been done conventionally. In the method disclosed in the '301 patent, two feed spools are wound each with half of the total length of optical fiber, and the first layer of fiber is wound on a planar side of a hollow disk-shaped coil form in a spiral pattern from the inner diameter of the disk to the outer diameter of the disk. The fiber is secured to the coil form with an adhesive. A second spiral layer is a mirror-image of the first, and is applied on the opposite planar side of the coil form. Subsequent spiral layers are wound such that the fiber loops which are positioned at equal distances from the center of the fiber optic coil are mirror images of the fiber loops on the opposite side of the thin hollow disk. This technique is intended to minimize the time-varying thermal gradients in the radial direction.
The invention disclosed in the '301 patent also provides for winding fiber layers in pairs and sequentially alternating the layer pairs, which are formed from fiber drawn from the two fiber feed spools, across each side of the thin coil form to minimize time-varying thermal gradients in the axial direction. Each fiber spiral pair has a layer wound from the inside to the outside, and another layer wound from the outside to the inside. The resultant coil therefore provides thermal symmetry, which greatly reduces both the radial and axial components of time-varying thermal gradients.
Although the winding method and fiber coil disclosed in the '301 patent is an improvement over the prior art, opportunities for substantial improvement over the prior art exist. One disadvantage of the winding method disclosed in the '301 patent is that subsequent layer of coils after the first layers on either side of the coil form are laid directly on top of the preceding layers, a practice that could result in slumps and sags of the optical fiber. In addition, because the layers are laid directly on top of one another, there are “contact points” between the contacting layers in each coil of fiber that are similar to the crossover sites in a conventional winding scheme, but which are subjected to much less stress than those crossovers. (See '301 patent, Col. 4 lines 13-30). However, it would be desirable if there were no such crossover or contact points. Further, errors in the winding pattern are rapidly compounded as layer count increases, as each previous layer directly affects the positioning of the subsequent layer.
Additionally, the process disclosed in the '301 patent starts the initial wind with the optical fiber passing through the center of the coil form with the winding performed from the inside to the outside on both sides of the coil form. To accomplish this in production, it would be necessary to either spool the fiber through the center hole in the coil form on the winder or cut a passage in the coil form from outer to inner diameters to allow pre-spooled fiber to pass through the coil form body to the center. Spooling the fiber through the center hole is not practical from a manufacturing standpoint when using an automated winding configuration, and cutting a passage in the coil form compromises the structural stability and symmetry of the coil form. It would therefore be desirable to have a winding method that does not require passing the pre-spooled fiber through the center of the coil form.